In Code Division Multiple Access (CDMA) wireless communications networks, data frames (“frames”) transferred by network elements often comprise errors that degrade the quality of information exchanged within such networks. Therefore, it is often necessary to identify errors within frames and to process the frames according to their error content so that frames of sufficient quality are selected to be passed on to higher layers in the network's communication protocol (“layers”). In general, the identification of errors within frames and the processing and transferring of frames is known as “frame selection”.
CDMA wireless networks support so-called “soft handoffs”. During a soft handoff, a primary frame and at least one copy of the primary frame, or “parallel” frame, are generated by different network elements. Before passing either the primary frame or the parallel frame on to higher layers in the network, it is necessary to determine which copy has superior quality. This is achieved by applying frame selection to each copy.
One example of a CDMA network is a Universal Mobile Telecommunications System (“UMTS”) network. A UMTS network is based on 5 MHz Wideband CDMA (“W-CDMA”) and is optimized for support of so-called “third generation services”, which include multimedia-capable mobile communications. FIG. 1 provides an exemplary diagram of a UMTS network 1.
Referring to FIG. 1, UMTS network “N” comprises core network 10, access network 20, mobile 30 (e.g., cellular telephone), Operations Administration and Maintenance (“OAM”) unit 40, Public Switched Telephone Network (“PSTN”)/Integrated Services Digital Network (“ISDN”) 50, and Internet/Intranet Service Provider (“ISP”) 60. Access network 20 comprises base stations 21a and 21b and network controllers or Serving Radio Network Controllers (“SRNCs”) 22a and 22b. Base stations 21a and 21b communicate with SRNC 22a or 22b via wired links (known as “IUB interfaces”) 3a and 3b. SRNCs 22a and 22b perform resource management functions and control base stations 21a and 21b, respectively, providing them access to core network 10. Frame selection units (“FSUs”) 23a and 23b are located within SRNCs 22a and 22b, respectively, and are responsible for performing frame selection. Mobile 30 communicates with base stations 21a and 21b via wireless links 1a and 1b. SRNCs 22a and 22b communicate with each other via a wired link (known as an “IUR interface”) 4 and communicate with the core network 10 via wired links (known as “IU interfaces”) 5a and 5b. Core network 10 has connections 6, 7 and 8 to OAM unit 40, PSTN/ISDN 50 and ISP 60, respectively.
In UMTS network N, mobile 30 transmits signals in the form of frames to base stations 21a and/or 21b. Frames are transmitted at successive, fixed time increments. A single frame comprises data bits that make up part of a complete signal. These frames also comprise “transport blocks”, which are basic units of data exchange. The size of a transport block (i.e., number of bits) is service-dependent. When a base station 21a or 21b receives a frame from mobile 30, the base station demodulates and decodes the frame to form a decoded frame. The base station 21a or 21b transfers the decoded frame to SRNC 22a or 22b at fixed time increments. Each frame comprises at least one decoded transport block and important information about the data within the transport block. Frames transferred to the SRNC 22a or 22b may comprise errors generated during transmission or decoding. FSUs 23a or 23b within SRNC 22a or 22b are adapted to apply frame selection to the transferred frames to analyze errors within the frames and process the frames according to their error content.
As noted above, frame selection is required when a mobile is in soft handoff (also called “diversity handover”). Soft handoff is designed to, among other things, enhance network quality by passing multiple copies of the same data to SRNCs. When mobile 30 is in a soft handoff mode, a primary base station 21a and at least one parallel base station 21b are in communication with the mobile 30. The primary base station 21a and parallel base station 21b both demodulate and decode a signal from the mobile 30. During a given time period, the primary base station 21a then transfers a primary frame to a primary SRNC 22a and then, if the parallel base station 21b is communicating with the primary SRNC 22a (which is not true in FIG. 1), the parallel base station 21b transfers a parallel frame (i.e., a frame, or “copy”, decoded during the same time period and comprising its own decoded version of the same transport block and data as the primary frame) to that same SRNC 22a over an IUB interface. If the parallel base station 21b is communicating with a different, or parallel SRNC 22b (as shown in FIG. 1), the parallel base station 22b transfers the parallel frame to the parallel SRNC and the parallel SRNC 22b then transfers the parallel frame to the primary SRNC 22a over an IUR interface 4. FSU 23a within the primary SRNC 22a is thereafter responsible for applying frame selection to the received frames.
Frame selection is not, however, limited to soft-handoff conditions. Even when a mobile 30 is not in a soft handoff mode, an FSU 23a or 23b must determine if the received frames are of satisfactory quality. If the received frames comprise errors, the FSU 23a or 23b has to determine what action needs to be taken. For example, some services provided by the network may be able to make use of partially errored frames. This requires the FSU 23a or 23b to determine whether the quality of a frame exceeds a minimum threshold with respect to the nature of errors within the frame. Based on the outcome, the FSU 23a or 23b must then decide if the frame is to be passed on to higher layers in the network or discarded.
Existing frame selection methods typically involve determining a “metric” which will represent frame quality with adequate accuracy. A variety of “hard” metrics (i.e., measures of frame quality which indicate whether a frame is “good” or “bad”) and “soft” metrics (i.e., measures of frame quality which indicate how good or bad a frame is based on a predetermined scale) have been studied. Cyclic redundancy check (“CRC”) codes are examples of error detection codes that are based on a hard metric. In cases where CRCs are not used, other methods of error detection are necessary. These methods typically involve the comparison of a soft metric of frame quality to a predefined threshold. Based on results from CRCs or soft metrics, if at least one received copy of a frame is detected correctly, then the FSU 23a or 23b simply selects the correctly detected copy and sends it to a higher layer in the network. However, if each received copy of a frame comprises errors, the FSU 23a or 23b must discard the received copies, or select a “best” copy to pass on. In the case where FSU 23a or 23b selects a best copy and CRCs are used, FSU 23a or 23b must randomly select a copy, because there is no way to tell the relative quality of the copies. CRCs will indicate that a frame is “bad” even when only a small part of a frame comprises errors. When soft metrics are used, FSU 23a or 23b cannot always select the copy having the highest quality because the results of soft metrics tend to be unreliable. The inability to tell the relative quality among copies of errored frames results in poor frame selection.
It has been observed that errors in frames that are generated in wireless networks tend to form “error bursts”. An error burst is a cluster, or burst, of errors that occur within a group of consecutive data bits in a frame. That is, error bursts are errors that tend to be concentrated within a group or multiple groups of consecutive bits, rather than scattered among singular bits. Errors occur in this manner because wireless channels vary slowly with time relative to typical data rates, and because of the inherent characteristics of convolutional code decoders or Turbo code decoders used in wireless networks. It is believed that knowledge of the fact that errors tend to be distributed in this manner can be used to provide more effective frame selection methods and systems.
Because errors often occur as error bursts, methods and systems for representing the location and length of errors to FSUs can be provided without using excessive amounts of IUB bandwidth. Furthermore, it is unlikely that error bursts will occur in the same parts of multiple frame copies. Therefore, it is believed that methods and systems can be provided for combining parts of frame copies to produce substantially error-free frames.
It is therefore a desire of the present invention to provide methods and systems for improving frame selection in wireless communication networks, based on the observation that errors in frames generated by wireless networks tend to form error bursts.